Russia has confirmed it used a deadly 'lung-busting' thermobaric bomb in Ukraine.
It is believed the weapon - often called a vacuum bomb - was used in the Chernihiv region, 80 miles north of Kyiv.
The area has been under fire for weeks and dozens of civilian deaths have been confirmed.
The UK Ministry of Defence says their use is not illegal but they are heavily regulated.
What are thermobaric weapons?
The weapons suck in oxygen from the surrounding air to generate a high-temperature explosion.
They typically produce a blast wave that lasts significantly longer than that of a conventional explosive and are capable of vapourising human bodies.
In warfare, they are often used in so-called ‘bunker buster’ roles, or to attack caves or tunnel systems.
The term thermobaric comes from the Greek ‘therme’ and ‘baros’ meaning ‘heat’ and ‘pressure’.
The MoD said they had been launched in Ukraine from a TOS-1A rocket launcher.
How thermobaric weapons work
The bombs detonate in two stages: first, fuel or dust-like material is dispersed into the air, forming a cloud.
Then the cloud is detonated to create an incredibly powerful shock wave.
The CIA says that such weapons can ‘obliterate’ people near the explosion and do significant damage to buildings.
What damage can they do to humans?
A CIA report says: “Those at the fringe are likely to suffer many internal, thus invisible injuries, including burst eardrums and crushed inner ear organs, severe concussions, ruptured lungs and internal organs, and possibly blindness.”
Dr Patricia Lewis, Research Director for International Security at Chatham House, told LBC that the wide area of the ignited particles make them 'suck in' oxygen in the first stage of the blast.
The blast from a thermobaric bomb is 'very sustained', Dr Lewis explains, and the weapons have become deadlier as the technology has developed.
The weapons were first developed in the 1960s by both the USA and the Soviet Union, and have been widely used in Afghanistan.
They are not illegal but they cannot be used against military targets in a way that may endanger the civilian population, or in a way that the damage they caused would be excessive to the military advantage gained.
Purposefully using it against civilians is illegal.
A thermobaric bomb dropped by US forces on the Taliban in 2017 left a crater 1,000 feet wide.
Launching indiscriminate attacks that kill or injure civilians constitutes a war crime.